Field of the Invention
The present invention relates to a zinc oxide piezoelectric crystal film which is grown on an R-plane sapphire substrate to orient its c-axis in parallel with the substrate surface, and more particularly, it relates to an improvement for attaining excellent orientation. Description of the Background Art
A typical example of a piezoelectric substrate for a surface wave device comprises a piezoelectric crystal film which is provided on a non-piezoelectric substrate. In relation to a surface wave substrate of such a type, known is a surface wave substrate which is formed by a nonpiezoelectric substrate of sapphire (.alpha.-Al.sub.2 O.sub.3) and a piezoelectric crystal film of zinc oxide (ZnO).
FIG. 2 shows a surface wave substrate 3 comprising a ZnO piezoelectric crystal film 2 which is provided on a sapphire substrate 1. In relation to such a surface wave substrate 3, it is known that a ZnO piezoelectric crystal film 2 of a (1120) plane is grown on the sapphire substrate 1 when the same is prepared from (0112) plane cut sapphire, i.e., R-plane sapphire. It is also known that the surface wave substrate 3 provides a high sound velocity and high electromechanical coupling factor when the crystal planes of ZnO and .alpha.-Al.sub.2 O.sub.3 are oriented as (1120)ZnO//(0112).alpha.-Al.sub.2 O.sub.3 and crystal axes are regularized as [0001]ZnO//[0111].alpha.-Al.sub.2 O.sub.3 as shown by arrows in FIG. 2, i.e., when the c-axis ([0001] direction) of ZnO is epitaxially grown in parallel with the substrate surface of the sapphire substrate 1.
In order to form a ZnO epitaxial film on sapphire, a process such as chemical transportation, CVD or sputtering is employed in general, while sputtering is widely employed among such processes, in particular.
When a pure ZnO thin film is formed on an R-plane sapphire substrate by sputtering, for example, its orientation is still insufficient for putting a piezoelectric substrate for a surface wave device which is formed by such a ZnO thin film into practice, although the c-axis in the ZnO thin film is regularized to some extent with respect to the substrate surface, i.e., a certain degree of orientation is attained. For example, the piezoelectric substrate is insufficient in piezoelectricity, and characteristics such as an electromechanical coupling factor are smaller than predetermined values.